Isolation And Characterization Of Natural Organic Matter In Coastal Environments

Natural organic matter (NOM) in the coastal environments, which include the sedimentary organic matters (SOM), particulate organic matters (POM), and dissolved organic matters (DOM), is an important component of global carbon cycle and plays important roles in the response to climate change. The bioavailable fraction of NOM can be consumed by heterotrophic microbes, and is an important component in the marine microbial cycle. The colored fraction of DOM is the principal chromophore in marine waters, influencing the color and temperature of surface waters. Marine NOM also plays key roles in metal and organic pollutants chelation, influencing metal and organic toxicity arid bioavailability.Due to the extremely complexity of marine NOM and the difficulty to acquire representative marine NOM samples, our knowledges for their properties and geochemical behaviors are still limited. This dissertation mainly focuses on the isolation and characterization of SOM and DOM in coastal environments. The sedimentary humic substances (SHS), dissolved humic substances (DHS) and DOM were isolated by using the methods established by the International Humic Substances Society and a novel coupled reverse osmosis-electrodialysis (RO/ED) method, respectively. To better understand the origin, composition, reactivity, and fate of the coastal NOM, elemental analysis, stable isotope analysis, Fourier transformed infrared spectroscopy (FTIR), thermogravimetric analysis (TGA),13C nuclear magnetic resonance spectroscopy (13C NMR), Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and potentiometric titrations were used to analyze those isolated samples. The different properties of marine DOM which were isolated by different methods were also compared and analyzed.The results of the SHS which were isolated from the estuarine and coastal sediments of the East China Sea as well as the mangrove sediments in the eastern coast of Hainan Island indicated that SHS represented a large fraction of SOM, with humin were the dominant fraction of SHS followed by humic acids and fulvic acids. Humic acids were found contain more nitrogen-containing compounds and highly branched long-chain aliphatic moieties, while fulvic acids contain more oxygen-containing functional groups, such as carboxyl and alcoholic groups. With the increasing distance from the shore and/or the increasing contribution from the marine source, the H/C ratios,δ13C values, as well as the content of aliphatic compounds, proteins, and carbohydrates increased, while the C/N ratios as well as the content of aromatic compounds and acidic functional groups decreased. As for the depth profile of SHS samples in the Changjiang estuary, it is found that the charge densities and aromatic compounds increased with increasing depth; however, the content of carbohydrates decreased with increasing depth. Therefore, it probably can be inferred that loss of labile carbohydrates and proteins and selective preservation of refractory lignin components may dominate the early diagenetic reactions of SHS in the Changjiang estuary. The average concentration of binding sites for the studied SHS is about22meq gC-1, and this value may provide insights for the species of metal cations in the sediments.The properties of DHS which were isolated from one mangrove pore-water sample and one near shore seawater sample downstream the mangrove-fringed estuary in the eastern coastal of Hainan Island indicated that the near shore seawater DHS enrich in13C and contain more aliphatic compounds, carbohydrates, but less aromatic structures and carboxyl groups than that of mangrove pore-water DHS. The significant mixing of photo-degraded mangrove DHS and the marine DHS were considered to the mechanism results in the difference for the two DHS samples. Combined with the δ13C values and C/N ratios of mangrove SHS in our previous study, it can be found both SOM and DOM in mangrove area are mainly from autochthonous mangrove detritus and can contribute to ambient environment in a large scale. The results of FTICR-MS indicated that there are big differences among the different fractions of DHS in the studied water samples, which may attribute to the different humification level of the different fractions. Fulvic acids may older than humic acids, while the hydrophilic XAD-4fraction may be the oldest fraction among DHS. The combination of the lignin degradation reaction and the Maillard condensation reactions are probably the formation and transformation mechanism of the studied DHS samples. The RO/ED method was used to isolate marine DOM from Barataria Bay and the coastal of Atlantic Ocean. The acid-base properties of the isolated marine DOM indicated that the concentration of carboxyl groups in marine DOM samples is about6.0～6.9meq gC-1, which is only60%of the carboxyl contents for soil and river NOM and fulvic acids. Phenolic hydroxyl groups could not be detected in the studied samples. Marine DOM in surface waters contains more carbohydrates, proteins, and lipids; however, marine DOM in deeper waters contains more aromatic compounds and condensed hydrocarbons. This phenomenon indicated that carbohydrates and proteins are reactive components, perhaps supporting much of the heterotrophic activity in the surface ocean. By comparing the properties of marine DOM isolated by RO/ED with that isolated by ultrafiltration and XAD resins which were reported in literatures, it can be found that the RO/ED method recovers both the carbohydrate-rich fraction of DOM that is isolated preferentially by UF and the fraction of DOM that is enriched in carboxyl groups and aliphatic moieties that is isolated preferentially by SPE methods. So the RO/ED method may recovery representative marine DOM from sea water.